Solid oxide fuel cell (SOFC) systems require ignition devices to ignite the air/fuel mixture in the fuel reformer during reformer warm-up and also the anode and cathode tail gases exiting the fuel cell stacks. Both the fuel cell reformer and the stacks operate in a very high temperature environment where temperatures can range from about 750° C. to about 1000° C., and typically are about 850° C. In SOFC applications, the ignition devices are completely contained in the high temperature environment, which is an atypical operating environment for prior art ignition devices adapted from other technologies. For example, diesel glow plugs and gasoline spark igniters are intended to withstand very high temperatures (>1000° C.) only at the very tip of the igniter element, while the rest of the device consists of materials, and is manufactured with processes intended to withstand temperatures much lower, typically in the range of about 300° C. to about 600° C. As a result, prior art ignition devices may fail to ignite after relatively short periods of time, such as about 10 hours to about 100 hours, when installed in an SOFC system. Since the working lifetime of an SOFC system at high temperature can be on the order of about 10,000 hours to about 40,000 hours, there is a need for an ignition device which can function reliably under sustained exposure to SOFC operating temperatures and can also reliably ignite a fuel/air mixture when an SOFC system cold re-start is commanded.
Prior art igniters fail in SOFC applications due to loss of electrical connectivity that occurs when the prior art materials used in the igniter undergo catastrophic oxidation due to the high temperature environment. Wire insulation becomes brittle and cracks, leaving the wire exposed to the environment. Prior art wire is formed of pure copper or pure nickel. Such wire cannot be used at temperatures above about 600° C. without suffering severe oxidation. Further, the prior art ceramic potting compound also fails due to a powdering phenomenon which leaves the igniter joint loose and susceptible to vibrations. Further, the prior art igniter braze re-melts, causing electrical disconnection of the metal clip to the igniter tip. Even further, the metal clips in a prior art design are made of a metal material which cannot withstand temperatures exceeding about 650° C.
What is needed in the art is a hot zone igniter capable of sustained service at temperatures consistent with the reformer combustion chamber and the anode stack gas of a solid oxide fuel cell system.
It is a principal object of the present invention to increase the working lifetime of a hot zone igniter for a fuel cell system.